There are many battery chemistries, LiFePO4 is but one .. the lithium batteries tend to be lighter in weight and more optimal for cars and mobility.

Grid storage has yet to find the cheapest storage per acre regardless of weight (which doesn't matter if they're not going to move) and solar -> thermal (underground) -> electricity (months later) is still being trialled (sort of, there's been decades of such usage at one site (IIRC)).